Most current mobile phones connect to a cellular network of base stations which are in turn interconnected to the public switched telephone network (PSTN). Global System for Mobile Communications (GSM) is the most popular standard for mobile phones in the world. GSM technology operates over a cellular network and due to certain technological advances is considered a second generation mobile phone system.
Although GSM differs significantly from its predecessor technologies with regard to signaling and speech channels, GSM is still vulnerable to basic forms of passive security attack, such as eavesdropping. This is mainly due to a signaling link within the fixed infrastructure part of the GSM signaling network which can expose users' unencrypted phone calls and data to an attacker if the attacker can manage to gain direct access to the signaling network.
GSM communications today are encrypted at the RF link between the cell phone and base station. The fixed link between the GSM base station and a mobile switching center providing the backbone of GSM network is however unencrypted.